Protease-dependent hemagglutinin cleavage contributes to alteration in chicken hemagglutination by the H3N2 influenza A virus.

: The human influenza A virus (H3N2) has been the predominant influenza strain since 1992, and one property of this virus is non-agglutination of chicken erythrocytes [Ch(-) virus]. The Ch(-) virus in our study was able to acquire chicken hemagglutination [Ch(+)] by trypsin passage but not by chymotrypsin passage. Moreover, the trypsin-passaged Ch(+) viruses reacquired the Ch(-) property after a further chymotrypsin passage. In particular, genetic analysis showed no evidence of mutations in the hemagglutinin (HA) gene during either trypsin or chymotrypsin passages: the only differences found were in the HA cleavage sites between the trypsin-passaged virus and the chymotrypsin-passaged virus as determined by the N-terminal amino acid sequence. These results suggested that protease-dependent differences at the viral HA cleavage site, rather than genetic mutations, are likely to have a significant effect on the viral ability to produce chicken hemagglutination.